He also participated in the first atomic mission that bombed Hiroshima on August 6, 1945. Flying as part of the crew of The Great Artiste which was a reference to him, purportedly because he could "hit a pickle barrel with a bomb from 30,000 feet" or he was "good with the fairer sex," his aircraft acted as the blast instrumentation support aircraft for the mission.

Beahan attended Rice University on a football scholarship during the 1930s. In 1942 he joined the Army Air Forces as an aviation cadet but washed out of pilot training, becoming a bombardier instead. He was assigned to the 97th Bombardment Group and took part in the first B-17 raids in Europe by Boeing B-17 Flying Fortresses. He flew 13 missions over Europe, 17 missions over North Africa, and five credited combat missions in the Pacific with the 509th Composite Group (including the Nagasaki sortie). He was shot down and crash-landed twice in North Africa. He returned to the United States as a bombing instructor in Midland, Texas. In the summer of 1944, he was recruited by Colonel Paul W. Tibbets to be part of the 509th Composite Group, which was formed to deliver the atomic bomb.

The mission to bomb Nagasaki was conducted on Beahan's 27th birthday. Admiral Frederick L. Ashworth, who participated on the mission as weaponeer, credited Beahan with saving the mission from failure by finding an opening in the clouds by which to complete the required visual bombing of the city. An estimated 35,000-40,000 people were killed outright by the bombing of Nagasaki, the majority of whom were munitions workers.

Following the Japanese surrender, he returned to the United States as a crewman in the record-breaking 1945 Japan–Washington flight under Lieutenant General Barney M. Giles. He remained in the Air Force until 1964, retiring as a lieutenant colonel. After his retirement, he worked as a technical writer for the engineering and construction firm Brown & Root through 1985.

In 1985, on the 40th anniversary of the Nagasaki bombing, Beahan said he would never apologize for the bombing, and that he had been thanked for his role by a group of 25 Japanese. He said the bombing was the "best way out of a hell of a mess."[1] Beahan hoped that he would forever remain the last man to have dropped an atomic bomb on people.